Ken Jenkins recently extracted a pair of images from the second security camera video at the Pentagon (the one without the obstruction) and put them together as you would see them in a blink comparator. A blink comparator is commonly used to detect subtle differences in astronomical photographs to be able to find variable stars, motions of asteroids, etc. The image of the plane is visible in the video frames long ago released by the Pentagon, but the plane is hard to see directly amid the clutter of the background. Only the smoke trail was obvious to most observers. Blinking the frames allows the plane to stand out from the background.
The real key to seeing the plane was to use source images as originally released under FOIA. Many of the copies that have been circulating on the internet for years are very degraded compared with these.